One wordly phenomenon, on my bucket list to one day observe, is the salmon runs in the Pacific Northwest. When you think about, most wild salmon species are pretty amazing. They undergo physiological changes that allow them to live in the two drastically different environments of fresh and saltwater, which classifies these fish as “anadromous.” One species who probably look forward to the annual freshwater return of the salmon more than any other is the grizzly bear.

Grizzly bears (who have a pretty fitting and intimidating scientific name of Ursus arctos horribilis) that live near the coast in British Columbia are known to enjoy the delectable delights of various Pacific salmon species (Oncorhynchus spp.) every year in the fall. Salmon are a beneficial addition to their diet, however, what the bears don’t know may hurt them. Salmon is a known “biovector” for mercury (a fact sushi lovers were probably aware of already), as it is bioaccumulative (increases in concentration through the food web) and toxic, with proven adverse effects on mammals’ immune and neurological systems if it is present in excess quantities. The global concern for mercury emissions has been on the rise, and in October of 2013, the Minimata Convention on Mercury was adopted as an international attempt to lower mercury emissions. A big problem is that it seems as though global mercury levels may still be increasing. East Asia emits almost 40% of the world’s total mercury emissions, and 15-24% of the deposition that occurs in the western United States comes from this region.

What they did

Figure 1 – A look at the levels of mercury observed in Grizzly Bear hair compared to a proposed level of neurological effects for polar bears.

A group of scientists from British Columbia and the Northwest United States put together a study investigating the potential uptake of mercury by grizzly bears by studying their hair. It turns out that hair is unique in its ability to resist any sort of chemical changes and incorporates mercury concentrations over time, making it a perfect matrix to study mercury contamination. How they did it Twenty wild grizzly bears were sampled from both the coast and the interior region of British Columbia to test for natural levels of mercury and five grizzly bears were the objects of a feeding trial study to see how mercury levels in hair respond to known doses of mercury. In the feeding study, the bears were fed Yellowstone Lake cutthroat trout for 33 days with varying levels of mercury higher than what’s typically found in their diet. The hair was shaved prior to the beginning of the feeding and sampled after the monitoring period was over when it was then tested for mercury with what’s called a “laser ablation inductively coupled mass spectrometer” (or LA-ICP-MS for the short and sane version).

What they found

The five bears that were fed fish with higher mercury levels than normal reflected the mercury intake in their hair. Mercury levels in the hair increased for 60 days, which was twice as long as the 33 day feeding period and increased by a range of 0.004 to 0.077 µg/g/day. Once the bears diet returned to normal, mercury levels decreased. Bears that consumed lower levels of mercury returned to background levels quicker than bears who consumed higher levels of mercury. The wild grizzly bears were separated into coastal and interior bears, as coastal bears have the potential to be affected by mercury levels in the salmon they are consuming. Interior bears had low mercury levels in their hair and there was little variation. Coastal bears differed and new hair growth showed an increase in mercury levels near the root of the hair, which reflected the bears’ switch in diet to salmon at the start of the salmon run period.

Results from bears sampled showed that 69% of those that consume salmon would have mercury levels above the threshold where subclinical neurological effects have been observed in polar bears (defined as 5.4 µg/g hair). While not a perfect comparison due to obvious differences between humans and Grizzly Bears, the researchers also compared the levels of daily mercury intake the bears were consuming to the daily human consumption threshold and found that around 93% of the bears are exceeding the daily mercury intake recommended for humans.

This study shows that in addition to providing bears with nutritional benefits, the salmon are also contributing to the mercury intake of these mammals. Grizzly Bears in Western Canada are considered a species of special concern and are also faced with other threats, such as habitat loss and climate change. The potential increase of mercury emissions globally is an additional concern these animals face.

Erin received her B.S. in Environmental Science from the University of Rhode Island in 2010 and is currently working towards her Masters at the University of Rhode Island’s Graduate School of Oceanography. Her current research involves persistent organic pollutants in the Atlantic Ocean.